1. Field of the Invention
The present invention relates to a power supply and more specifically relates to PWM control of a switching mode power supply.
2. Description of Related Art
Power supplies have been used to convert an unregulated power source to a regulated voltage or current. FIG. 1 shows a traditional power supply, in which a PWM controller 10 generates a switching signal for switching a transformer 11 via a transistor 20. The duty cycle of the switching signal determines the power delivered from an input of the power source to an output of the power supply. Although the switching technology reduces the size of the power converter, switching devices generate electric and magnetic interference (EMI) that interferes the power source. An EMI filter 15 equipped at an input of the power supply is utilized to resist the EMI into the input of the power source. However, the EMI filter 15 used to reduce the EMI causes power consumption and increases the cost and the size of the power supply. In recent development, many prior arts have been proposed to reduce the EMI by using frequency jitter. They are, “Effects of switching frequency modulation on EMI performance of a converter using spread spectrum approach” by M. Rahkala, T. Suntio, K. Kalliomaki, APEC 2002 (Applied Power Electronics Conference and Exposition, 2002), 17-Annual, IEEE, Volume 1, 10–14, Mar., 2002, Pages: 93–99; “Offline converter with integrated softstart and frequency jitter” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,229,366, May 8, 2001; and “Frequency jittering control for varying the switching frequency of a power supply” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,249,876, Jun. 19, 2001. However, the disadvantage of these prior arts is that the frequency jitter generates undesired ripple signal at the power supply outputs. The undesired ripple signal generated by the frequency jitter could be realized as following description. An output power PO of the power supply is the product of an output voltage VO and an output current IO, which is given by,PO=VO×IO=η×PIN  (1)
An input power PIN of the transformer 11 and a switching current IP can be respectively expressed as,
            P      IN        =                  1                  2          ×          T                    ×              L        P            ×              I        p        2                        I      P        =                            V          IN                          L          P                    ×              T        ON            Where η is the efficiency the transformer; VIN is the input voltage; LP is the primary inductance of the transformer 11; T is the switching period of the switching signal; TON is the on-time of the switching signal.
The equation (1) can be rewritten as,
                              P          O                =                  η          ×                                                    V                IN                2                            ×                              T                ON                2                                                    2              ×                              L                P                            ×              T                                                          (        2        )            
The switching period T varies in response to the frequency jitter. As shown in equation (2), the output power PO will vary in response to the variation of the switching period T. The variation of the output power PO therefore generates an undesired ripple signal.
An object of the present invention is to provide a PWM controller having frequency jitter to reduce the EMI for power supply. The frequency jitter will not generate the ripple signal at the power supply outputs. Another object of the present invention is to reduce the complexity and the cost of the circuit that generates the frequency jitter.